Semiconductor optical waveguides have been fabricated in the Group III-V systems by cladding an epitaxial layer of high refractive index material with one or more epitaxial or mass transported layers of lower refractive index material.
For example, in the InGaAsP/InP system, a quaternary (InGaAsP) guiding layer is clad with lower index binary (InP) layers to form an optical waveguide. While the As content of the quaternary guiding layer can be raised to increase the refractive index of guiding layer with respect to the cladding layers, this decreases the energy bandgap of the guiding layer and, thereby, increases the cutoff wavelength of the waveguide. As such, the resulting waveguide exhibits a limited transparency range in the window of wavelengths (1.3 to 1.6 .mu.m.) of interest in lightwave communications. In addition to this limitation, quaternary layers such as InGaAsP are difficult to grow using procedures such as MOVPE because such growth requires precise, stable control of temperatures and flows of precursor materials used in the growth process.
Other techniques have been employed to produce these semiconductor optical waveguides. For example, extremely thin layers of InP and InGaAs have been interleaved to create an "effective" quaternary guiding layer. While this approach is similarly limited with respect to transparency, it introduces a further requirement in the growth process, namely, the need for a manifold capable of switching rapidly between different precursor materials to create abrupt interfaces.